This invention relates to variable delay techniques and, more particularly, to a selectable delay system. The subject matter of this invention is related to subject matter disclosed in my copending U.S. application Ser. No. 000,430 entitled "Variable Delay System", filed of even date herewith and assigned to the same assignee as the present application. The invention is especially useful in ultrasonic imaging systems.
In recent years ultrasonic techniques have become more prevalent in clinical diagnosis. Such techniques have been utilized for some time in the field of obstetrics, neurology and cardiology, and are becoming increasingly important in the visualization of subcutaneous blood vessels including imaging of smaller blood vessels.
Various fundamental factors have given rise to the increased use of ultrasonic techniques. Ultrasound differs from other forms of radiation in its interaction with living systems in that it has the nature of a mechanical wave. Accordingly, information is available from its use which is of a different nature than that obtained by other methods and it is found to be complementary to other diagnostic methods, such as those employing X-rays. Also, the risk of tissue damage using ultrasound appears to be much less than the apparent risk associated with ionizing radiations such as X-rays.
The majority of diagnostic techniques using ultrasound are based on the pulse-echo method wherein pulses of ultrasonic energy are periodically generated by a suitable piezoelectric transducer such as a lead zirconate-titanate ceramic. Each short pulse of ultrasonic energy is focused to a narrow beam which is transmitted into the patient's body wherein it eventually encounters interfaces between various different structures of the body. When there is a characteristic impedence mismatch at an interface, a portion of the ultrasonic energy is reflected at the boundary back toward the transducer. After generation of the pulse, the transducer operates in a "listening" mode wherein it converts received reflected energy or "echoes" from the body back into electrical signals. The time of arrival of these echoes depends on the ranges of the interfaces encountered and the propagation velocity of the ultrasound. Also, the amplitude of the echo is indicative of the reflection properties of the interface and, accordingly, of the nature of the characteristic structures forming the interface.
There are various ways in which the information in the received echoes can be usefully presented. In one common technique, the electrical signals representative of detected echoes are amplified and applied to the vertical deflection plates of a cathode ray display. The output of a time-base generator is applied to the horizontal deflection plates. Continuous repetition of the pulse/echo process in synchronism with the time-base signals produces a continuous display, called an "A-scan", in which time is proportional to range, and deflections in the vertical direction represent the presence of interfaces. The height of these vertical deflections is representative of echo strength.
Another common form of display is the so-called "B-scan" wherein the echo information is of a form more similar to conventional television display; i.e., the received echo signals are utilized to modulate the brightness of the display at each point scanned. This type of display is found especially useful when the ultrasonic energy is scanned transverse the body so that individual "ranging" information yields individual scanlines on the display, and successive transverse positions are utilized to obtain successive scanlines on the display. The technique yields a cross-sectional picture in the plane of the scan, and the resultant display can be viewed directly or recorded photographically or on magnetic tape. The transverse scan of the beam may be achieved by a reflector which is scanned mechanically over a desired angle.
Two types of focusing techniques are most prevalent in ultrasonic imaging equipment; i.e., fixed focus and dynamic focus. In a fixed focusing technique, the returning echoes are assumed to originate from a particulate range in the body being imaged. Based on this assumption, appropriate delays are imparted to the different portions of the returning ultrasound beam so that all portions of the beam arriving at the transducer from the particular range can be added approximately in phase. The focusing may be achieved, for example, by providing a focusing lens (as described in the U.S. Pat. No. 3,958,559), by providing a segmented transducer in conjunction with appropriate fixed delays, or by combinations of these or other techniques.
Dynamic focusing techniques typically utilize a segmented transducer and the signals received at the transducer segments are summed through appropriate variable delays to obtain different effective foci as a function of time. As the investigating beam moves deeper into the body, the variable delays are varied appropriately to move the effective focus deeper into the body.
Fixed focus systems are advantageous in that they are less complex and less expensive than their dynamically focused counterparts. However, when it is desired to have an equipment which can operate over a substantial range of depth in a body, a fixed focus system may be inadequate. However, in such cases a full dynamic focusing capability may not be required and could involve undue complexity and expense.
It is one of the objects of the present invention to provide an ultrasonic imaging apparatus which includes a selectable focus having performance advantages as compared to fixed focus techniques, but which is less complex than prior art dynamic focusing techniques.